Chemical mechanical polishing composition and process

ABSTRACT

A composition for chemical mechanical polishing includes a slurry. A sufficient amount of a selectively oxidizing and reducing compound is provided in the composition to produce a differential removal of a metal and a dielectric material. A pH adjusting compound adjusts the pH of the composition to provide a pH that makes the selectively oxidizing and reducing compound provide the differential removal of the metal and the dielectric material. A composition for chemical mechanical polishing is improved by including an effective amount for chemical mechanical polishing of a hydroxylamine compound, ammonium persulfate, a compound which is an indirect source of hydrogen peroxide, a peracetic acid or periodic acid. A method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material. A selectively oxidizing and reducing compound is applied to produce a differential removal of the metal and the dielectric material. The pH of the slurry and the selectively oxidizing and reducing compound is adjusted to provide the differential removal of the metal and the dielectric material. A method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of a hydroxylamine compound, ammonium persulfate, a compound which is an indirect source of hydrogen peroxide, a peracetic acid or periodic acid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Patent Application Ser.No. 60/023,299 filed on Jul. 25, 1996 under Title 35, United StatesCode, Sections 111(b), and claims the benefit thereof under Title 35,United States Code, Section 119(e).

INTRODUCTION

1. Technical Field

This invention relates to an improved composition and process for thechemical mechanical polishing or planarization of semiconductor wafers.More particularly, it relates to such a composition and process whichare tailored to meet more stringent requirements of advanced integratedcircuit fabrication.

2. Background

Chemical mechanical polishing (or planarization) (CMP) is a rapidlygrowing segment of the semiconductor industry. CMP provides globalplanarization on the wafer surface (millimeters in area instead of theusual nanometer dimensions). This planarity improves the coverage of thewafer with dielectric (insulators) and metal substrates and increaseslithography, etching and deposition process latitudes. Numerousequipment companies and consumables producers (slurries, polishing pads,etc.) are entering the market.

CMP has been evolving for the last ten years and has been adapted forthe planarization of inter-layer dielectrics (ILD) and for multilayeredmetal (MLM) structures. During the 80's, IBM developed the fundamentalsfor the CMP process. Previously (and still used in many fabs today)plasma etching or reactive ion etching (RIE), SOG ("spin on glass"), orreflow, e.g., with boron phosphorous spin on glass (BPSG), were the onlymethods for achieving some type of local planarization. Globalplanarization deals with the entire chip while "local" planarizationnormally only covers a ˜50 micron² area.

At the 1991 VMIC Conference in Santa Clara, Calif., IBM presented thefirst data about CMP processes. In 1993 at the VMIC Conference, IBMshowed that a copper damascene (laying metal lines in an insulatortrench) process was feasible for the MLM requirements with CMPprocessing steps. In 1995 the first tungsten polishing slurry wascommercialized.

The National Technology Roadmap for the Semiconductor Industries (1994)indicates that the current computer chips with 0.35 micron feature sizeswill be reduced to 0.18 micron feature size in 2001. The DRAM chip willhave a memory of 1 gigabit, and a typical CPU will have 13 milliontransistors/cm² (currently they only contain 4 million). The number ofmetal layers (the "wires") will increase from the current 2-3 to 5-6 andthe operating frequency, which is currently 200 MHz, will increase to500 MHz. This will increase the need for a three dimensionalconstruction on the wafer chip to reduce delays of the electricalsignals. Currently there are about 840 meters of "wires"/chip, but by2001 (without any significant design changes) a typical chip would have10,000 meters. This length of wire would severely compromise the chip'sspeed performance.

The global planarization required for today's wafer CDs (criticaldimensions) improves the depth of focus, resulting in better thin metalfilm deposition and step coverage and subsequently increases waferyields and lowers the cost/device. It is currently estimated (1996) thatit costs $ ˜114/layer/wafer with current limited planarizationprocesses. As the geometries become smaller than 0.35 micron, theplanarity requirements for better lithography become critical. CMP isbecoming important, if not essential, for multiple metal levels anddamascene processes.

The CMP process would appear to be the simple rotation of a wafer on arotary platen in the presence of a polishing medium and a polishing padthat grinds (chips away) the surface material. The CMP process isactually considered to be a two part mechanism: step one consists ofchemically modifying the surface of the material and then in the finalstep the altered material is removed by mechanical grinding. Thechallenge of the process is to control the chemical attack of thesubstrate and the rate of the grinding and yet maintain a highselectivity (preference) for removing the offending wafer featureswithout significant damage to the desired features. The CMP process isvery much like a controlled corrosion process.

An added complexity is that the wafer is actually a complex sandwich ofmaterials with widely differing mechanical, electrical and chemicalcharacteristics, all built on an extremely thin substrate that isflexible.

The CMP processes are very sensitive to structural pattern density whichwill affect metal structure "dishing" and oxide erosion. Large areafeatures are planarized slower than small area features.

At the recent SEMICON/Southwest 95 Technical program on CMP, it wasstated that "Metal CMP has an opportunity to become the principalprocess for conductor definition in deep submicron integrated circuits."Whether or not it does so depends on the relative success of CMPtechnologists in achieving the successful integrated process flow atcompetitive cost.

Slurries: CMP has been successftilly applied to the planarization ofinterdielectric levels (IDL) of silicon oxides, BPSG, and siliconnitride and also metal films. The metal films currently being studiedinclude tungsten (W), aluminum (Al) and copper (Cu).

The polishing slurries are a critical part of the CMP process. Thepolishing slurries consist of an abrasive suspension (silica, alumina,etc.) usually in a water solution. The type and size of the abrasive,the solution pH and presence of (or lack of) oxidizing chemistry arevery important to the success of the CMP process.

Metal CMP slurries must have a high selectivity for removing theunwanted metal compared to the dielectric features on the wafers. Themetal removal rate should be between 1700 to 3500 Å/min) withoutexcessive "dishing" of the metal plugs or erosion of the oxidesubstrate.

The oxide CMP has similar requirements and polishing rates close to 1700Å/minute.

Metal Polishing: This type of polishing relies on the oxidation of themetal surface and the subsequent abrasion of the oxide surface with anemulsion slurry. In this mechanism, the chemistry's pH is important. Thegeneral equations are (M=metal atom):

    M.sup.o →M.sup.n+ +ne.sup.-

    M.sup.n+ +[Ox].sub.y →MO.sub.y or [M(OH).sub.x]

Under ideal conditions the rate of metal oxide (MO_(y)) formation(V_(f)) will equal the rate of oxide polishing (V_(p)), (V_(f) =V_(p)).If the pH is too low (acidic) then the chemistry can rapidly penetratethe oxide and attack the metal (V_(f) <V_(p)), thus exposing the metalwithout any further oxide formation. This means that all metal surfaces,at high points and in valleys, are removed at the same rate.Planarization of the surface is not achieved. This could cause metalplug connectors to be recessed below ("dishing") the planarizationsurface which will lead eventually to poor step coverage and possiblepoor contact resistance.

When the pH is too high (caustic), then the oxide layer may becomeimpenetrable to the chemistry and the metal becomes passive, (V_(f)>V_(p)) and the metal polishing rate becomes slow. Metal polishingselectivity to oxide generally ranges from 20 to 100:1, depending on themetal type. Tungsten metal should have selectivities >50:1 for the metalto oxide, and copper could have >140:1 metal to oxide selectivity. Etchrates can be up to 7000Å/min. The chemical diffusion rate and the typeof metal oxide surface are important to the successful planarizationprocess. A detailed mechanism has been proposed by Kaufman.

In practice, the low pH and highly corrosive oxidants (ferric nitrate)being used with an example metal CMP process has created corrosionproblems with the polishing equipment. Currently the oxidant used in themetal polishing step has ranged from nitric acid to hydrogen peroxide,cesium and ferric nitrate solutions and even ferric cyanide solutions.Because of chemical stability problems, many slurries are made up at thepoint of use which means that there is little or no shelf life.

Metal planarization needs an oxidizing reagent that is stable and is notgoing to contribute to mobile ion contamination, will not "stain" theequipment, will not affect the slurry composition and slurry particledistribution and is generally environmentally friendly. The currenthydrogen peroxide systems are not stable when premixed with the slurryand therefore have to be delivered to the polishing equipment withseparate pumping systems and mixed at the point of use. The ferricnitrate system requires a low pH and is known to "stain" the polishingequipment. The potassium iodate system also requires special handling.

An emerging area of CMP will deal with the copper damascene process. Thecopper metal interconnects (wires) will be required because of itsbetter conductivity compared to Al. One major disadvantage with copperis its easy diffusion through silica under normal operating conditions.The copper damascene process will need barrier layers to prevent thiscopper diffusion.

In the damascene process, "lines" or trenches are etched into theinterdielectric layers, and then the walls of these trenches are coatedwith barrier materials. These materials can be composed of Ta, TaN, Tior TiN among other materials. Copper metal is then deposited, byelectroless or electrode plating, or PVD or CVD methods. The excesscopper above the trench is then removed by chemical mechanicalpolishing. The difficult part of the CMP process is not to remove excesscopper ("dishing") which will remove the copper metal below theinterdielectric layer.

CMP of the copper metal can be done over a wide pH range (2 to 12).Pourbaix diagrams for copper indicate that copper can only be passivated(oxide layer) in neutral or basic solutions. In acid solutions aninhibitor, i.e., benzotriazole (BTA) is usually needed to control theisotropic etching effects from the chemistries used in the CMP process.Much of the CMP work has been done with hydrogen perioxide at various pHranges.

Some CMP work has been done with ammonium hydroxide, because of itsability to form copper complexes though there are problems with poorselectivity between copper and titanium and silicon oxide.

Interlayer Dielectric (Oxide) Polishing: Recently a group of engineersusing ILD (oxide) CMP was asked to prioritize CMP processingrequirements. The major concern was surface damage (scratching, etc.)followed by wafer (polishing) nonuniformity (within wafer and wafer towafer), then polishing rate and finally planarity. The mechanisms arestill being developed, but the polishing process appears to involve twoconcurrent processes; a mechanical process involving plastic deformationof the surface and, chemical attack by hydroxide (⁻ OH) to form silanolbonds. ##EQU1##

In a slurry (colloidal suspension) the pH is important and for thesilicon oxide system it needs to be in the 10 to 11.5 range. CurrentlyCMP users are using silicon oxide-based slurries which were "buffered"with sodium hydroxide but now are being formulated with potassium orammonium hydroxide solutions. Etch rates can be in the range of 1700Å/min.

If the pH is too high the polynuclear species may start to precipitatein an unpredictable manner. There is also the possibility of ancondensation process to form Si bonds.

There are other important features of the silicon surface that willinfluence the etch rates and final surface conditions; (metalcontamination and possibly micro scratches). As mentioned above thetypical silicon surface is terminated (covered) with --OH groups underneutral or basic conditions. The silicon surface is hydrophilic (thesurface is "wettable"). These groups activate the surface to a number ofpossible chemical or physioabsorbtion phenomena. The Si--OH groupsimpart a weak acid effect which allows for the formation of salts and toexchange the proton (H⁺) for various metals (similar to the ion exchangeresins). These Si--O⁻ and Si--OH can also act as ligands for complexingAl, Fe, Cu, Sn and Ca. Of course the surface is very dipolar and soelectrostatic charges can accumulate or be dissipated depending on the,bulk solution's pH, ion concentration and charge. This accumulatedsurface charge can be measured as the Zeta potential.

If the silica (Si) surface underneath the oxide layer is exposed becauseof an over aggressive polishing process, this could causeelectrochemical problems because silica has a modest redox potentialwhich will allow Cu, Au, Pt, Pb, Hg and Ag to "plate on" the silicasurface. Exposure to light will also affect the redox reaction for Cu.The light will "generate" electrons in the semiconductor Si materialwhich then reduces the copper ion to Cu^(o).

Post-Clean Processes: Both the ILD and metal polishing processes musteventually pass through a final cleaning step to remove traces of slurryand the chemistry. Though the process appears to be simple, i.e. a brushscrub and a rinse cycle, considerable effort is being expanded todetermine if the process should involve either single side, double sidedscrubbing, single wafer or batch processing, spray tools or evenimmersion tanks. Recently an engineering group working with post-cleanCMP ranked wafer cleanliness (from slurry and pad particles and metalliccontamination) as the most important issue in the post-clean step.Process reliability and defect metrology were the other two importantareas of concern.

Residual particle levels must be ˜1 particle/20cm², and 90% of theseparticles with less than >0.2 micron size. Line widths of 0.35 micronwill require the removal of particles down to 0.035 or less. Incompleteparticle removal will decrease wafer yield. Low defect (scratches)levels and acceptable planarity will also be very important.

Most fabs have developed their own in-house technology for thepost-clean CMP steps. Most of the "chemistries" involve DI water witheither added ammonium hydroxide or HF while some fabs are using thestandard RCA SC-1 (NH₄ OH:H₂ O₂ :H₂ O) and SC-2 (HC:H₂ O₂ :H₂ O)cleaning steps traditionally used in the front end process.

There are five mechanisms for removing impurities (particles and/orions) from wafer surfaces:

Physical desorption by solvents: Replacing a small number of stronglyabsorbed material with a large volume of weakly adsorbed solvent(changing the interaction of the surface charges).

Change the surface charge with either acids or bases: The Si--OH orM--OH group can be protonated (made positive) in acid or made negativewith bases by removing the proton.

Ion competition: Removing adsorbed metal ions by adding acid (i.e. ionexchange).

Oxidation or decomposition of impurities: Oxidation of metals, organicmaterials or the surface of slurry particles will change the chemicalbonds between the impurities and substrate surface. The chemicalreaction can either be through redox chemistry or free radicals.

Etching the surface: The impurity and a certain thickness of thesubstrate surface is dissolved.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, a composition forchemical mechanical polishing includes a slurry. A sufficient amount ofa selectively oxidizing and reducing compound is provided in thecomposition to produce a differential removal of a metal and adielectric material. A pH adjusting compound adjusts the pH of thecomposition to provide a pH that makes the selectively oxidizing andreducing compound provide the differential removal of the metal and thedielectric material.

In accordance with a second aspect of the invention, a composition forchemical mechanical polishing is improved by including an effectiveamount for chemical mechanical polishing of a hydroxylamine compound.

In accordance with a third aspect of the invention, a composition forchemical mechanical polishing is improved by including ammoniumpersulfate.

In accordance with a fourth aspect of the invention, a composition forchemical mechanical polishing is improved by including a compound whichis an indirect source of hydrogen peroxide.

In accordance with a fifth aspect of the invention, a composition forchemical mechanical polishing is improved by including a peracetic acid.

In accordance with a sixth aspect of the invention, a composition forchemical mechanical polishing is improved by including periodic acid.

In accordance with a seventh aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material. A selectively oxidizing and reducingcompound is applied to produce a differential removal of the metal andthe dielectric material. The pH of the slurry and the selectivelyoxidizing and reducing compound is adjusted to provide the differentialremoval of the metal and the dielectric material.

In accordance with an eighth aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material, and an effective amount for chemicalmechanical polishing of a hydroxylamine compound.

In accordance with an ninth aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material, and an effective amount for chemicalmechanical polishing of ammonium persulfate.

In accordance with a tenth aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material, and an effective amount for chemicalmechanical polishing of a compound which is an indirect source ofhydrogen peroxide.

In accordance with an eleventh aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material, and an effective amount for chemicalmechanical polishing of a peracetic acid.

In accordance with an twelfth aspect of the invention, a method forchemical mechanical polishing comprises applying a slurry to a metal anddielectric material surface to produce mechanical removal of the metaland the dielectric material, and an effective amount for chemicalmechanical polishing of periodic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are Pourbaix diagrams for copper and metal, useful for anunderstanding of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Now CMP appears to be entering a new growth phase, which emphasizes anew group of priorities. These priorities include reducing CMP defectsin metal and insulator layers, better planarity within wafer and waferto wafer, a premixed concentrate that avoids point of use mixing; ageneric post CMP cleaning and a high polishing selectivity. There arealso environmental, health and safety issues. These issues are: (1)better vapor handling (or reduced requirement for vapor handling), (2)possible slurry recycling (or spent slurry residues that are moreenvironmentally friendly), (3) more stable chemistries to be used withthe abrasives and (4) better end point detection (EPD) during thepolishing steps.

This invention does not deal with the composition or type of abrasive(slurry particle size, shape, size distribution, % solids) in theslurry. But the slurries have numerous other components (oxidizingagents, stabilizers, etc.) that can be improved through additionalexperiments. These components include solution pH, type of chemistry andchemical and slurry purity. This proposed invention focuses on thechemistry and its possible pH, Zeta potential, contact angle ("wetting")and other associated effects.

The first phase of the invention focuses on understanding the CMPchemistry based on hydroxylamine (HDA) and hydroxylamine derivatives(the chloride, sulfate, nitrate or other salts) under different pHconditions. HDA (NH₂ OH) can be viewed a hybrid between hydrazine (NH₂NH₂) and hydrogen peroxide (H₂ O₂) in its redox chemistry. HDA is a moreselective (controllable) oxidation and reducing agent. This dualcapability is achieved by shifting the pH from the acid to basic media,i.e. ##EQU2##

The redox potential for hydrogen peroxide (acidic) and HDA (in acid andbase) (E_(v) at SHE) are given: ##EQU3##

Fortunately few metal ions are reduced to the zero oxidation state, andthis is important in CMP processes to avoid contamination of the wafersurface with metal particles. Hydrogen peroxide polishing systems arealso not very stable, being easily decomposed by trace amounts oftransition metals. Currently, the CMP consumable suppliers need to havea two component delivery system--one for the slurry and the second forthe peroxide.

Besides being a redox agent, HDA, like ammonia, can form complex saltswith many metals including Al(SO₄)₂ *NH₂ OH* H₂ O and Cu (x)₂ *NH₂ OH*H₂ O.

Another important advantage of using hydroxylamine type compounds istheir decomposition products. Depending on the solution pH and metalions and concentration, HDA will decompose to water, nitrogen, ammoniaand N₂ O. The formation of nitrogen even takes place through a slowinternal redox reaction at pHs above 8.

Metal Polishing: The metals currently being studied for the CMP processinclude AI, Cu, and W. Pourbaix diagrams can be used to examine the bestregions (E_(v) versus pH) for the various polishing rates (corrosion).No two metal or alloy systems will have the same regions of chemicalactivity. Using this data may also allow CMP polishing conditions to bechosen so that the selectivity of the polishing rate of one metal issignificantly greater than another metal (or oxide or nitride material)on the same wafer. Pourbaix diagrams can be obtained for all metals,oxides, nitrides and other materials appearing on wafer surfaceswherever they are available. By overlaying the diagrams, pH regions canbe roughly determined which may be corrosive for one material whilepassivating for another. This could be one tool that is useful inseeking high selectivities. FIG. 1 shows the Pourbaix diagram for Cu.This diagram, based on thermodynamic data, shows that copper, copper (I)oxide and copper (II) oxide can exist together in the redox environmentof our world (delineated by the sloping parallel dashed lines). The dataalso shows that none of these three compounds can exist at pHs less than˜6.8, and at oxidation potentials above ˜0.2 volts, all of thesecompounds will dissolve.

At higher pH values the three compounds can exist in aqueous solution,including with various anions (Cu(OH)₂ and CuO₂.

This invention proposes that usage of HDA or its salts can be used toremove copper using CMP methods. The advantage of using the HDA basedchemistries is that its oxidation potential (E_(v) =˜1.05 volts) willallow the Cu to be removed at higher pHs than conventional chemistriesthat require a more acidic environment (lower pH).

Recent experiments with 10% hydroxylamine nitrate in DI water showedthat 3000 A copper metal on a 300 A Ti metal layer could be cleanlyremoved; @ pH 3˜100 A/min, pH 4˜125 A/min and pH 5˜1000 A/min. This isexactly the reverse of the expected pH effect from the Pourbaix diagramand is the result of the oxidation potential.

When the free base hydroxylamine (5% in DI water) was tested with thesame type of copper wafer, the etching rate dropped to 75 A/min comparedto a 10% ammonium hydroxide with a 100 A/min rate. It is known thatammonium hydroxide solutions will dissolve copper very slowly, but ifoxidizing agents (air or oxygen) are introduced then the etching ratecan be quite measurable. The hydroxylamine solution is a reducing mediumand so the copper etch is slower. The data does show that HDA could beused for very controlled (slow) etch rates.

FIG. 2 shows the Pourbaix diagram for aluminum metal. The data showsthat the pure metal Al cannot exist in the normal redox regime but onlyas an oxide coating. Between a pH of 4 and 10 this oxide layer will notdissolve.

Experiments with blanket Al metal wafers should again show the Al metaland its oxide layer can be removed by using either HAN at a pH of 4 orat 10 since it is necessary to remove the oxide layer before the metallayer can be polished. Concentration ranges will vary from 0.5 to 10 wt%.

Our understanding of HDA and its purification has given us a uniqueunderstanding of HDA's capacities to aid in removing mobile ions(sodium, potassium, iron and other transition metal ions) from thewafer's surface. It is critical that all phases of the CMP processminimize the mobile and transition metal ion concentrations on the wafersurfaces.

It is possible to add chelating agents; i.e. alkyl beta-diketones (2,4pentanedione, etc.) or EDTA or aromatic phenolic aldehydes(salicylaldehyde, etc.) or other agents. These components can be addedin concentrations ranging from 2 ppm to 15 wt %. Higher concentrationscould be used but there is a possibility that these chelators could"plate" on the chip's structures, or would alter the effectiveness ofthe over all chemistry. The ketone-based systems may react with thehydroxylamine based products to form oxime derivatives which are goodchelating agents in their own right.

Other agents could include bis(hydroxypropyl)hydroxylamine, anisaldehydeor even alpha hydroxy isobutyric acid as a chelator. Other compoundscould also be aromatic dioxygenated compounds, benzoin and benzil.

A recently reported water soluble iron chelator is O-TRENSOX which canbe used in the HDA-based chemistries and should show promising results.

Though catechol and catechol derivatives are known to be good chelatingagents at high pH conditions (because of the mono or dianion) only alittle work has been done with this class of compounds under acidicconditions. There are reports that catechol will complex with aluminumat pH 3-5.

Gallic acid is also another compound that under mildly acidic conditionscould have complexing powers with certain Group 3 through 12 metals(IUPAC nomenclature). The catechol and gallic acid family of compoundscan act as either corrosion inhibitors (at "high" concentrations; i.e.0.5 to 15-20 wt %) compounds, or as metal chelators in the ppm to 0.5 wt% range.

For many oxygenated compounds (phenols, alcohols, some organic acids,etc.) it is important that the oxygen atoms fill in vacancies on themetal surfaces. These vacancies are formed because of poorly organizedsurface oxide films and/or the pH retards the reactions or other anionsinterfere with the film uniformity. If the chemical environment is tooaggressive then the corrosion inhibitor that is absorbed on the surfacewill be dissociated from the surface, but will carry a metal ion withit. Now the corrosion inhibitor can give the appearance of a attackingspecies.

Other benefits to using HDA-based chemistries are the environment,safety and health aspects. HDA under basic conditions decomposes towater, nitrogen, and small concentrations of NH₃. HDA is mildly causticcompared to other nitrogen containing compounds, i.e., organic amines.Under acidic conditions, hydroxylamine compounds are very stable inaqueous solutions.

CMP users do not like working with sodium or potassium hydroxide becauseof the potential mobile ion contamination. Many users have changed overto ammonium hydroxide which does not have the same magnitude of a mobileion problem and does have a lower surface tension (better surfacecontact). The main problem with ammonium hydroxide is its odor whichrequires very effective ventilation systems.

Another important area is to understand and, if possible, to adjust theslurries' Zeta potential. The Zeta potential is a electrostaticpotential measurement of the interaction of the electrostatic doublelayer ions (anions and cations) that exists around each particle in asolution. The Zeta potential depending on the type of particle; i.e.aluminum, silica, manganese dioxide etc., and the solution pH, can bepositive or negative. Poorly designed slurries may have a Zeta potentialwhich leads to settling of the slurry particles. This can be verydetrimental to its performance during the CMP polishing process.

Another measure of Zeta potential is the isoelectric point (IEP) for aparticle. The IEP is the pH at which the Zeta potential value is zero.The chemical composition and source will have significant effect on theIEP. Some selected values: aluminum oxide particles can vary between 3.8to 9.4, while silicon oxide has a narrower range 1.5 to 3.7.

Some metal residue IEP values are 9.5 for TiO₂, while tungsten issomewhere around ˜1. Such wide ranges of values pose a major challengeto developing chemistries to control the Zeta potential of the particlesthat may eventually adhere to the wafer surface.

Another concern is that the Zeta potential between the slurry and metalparticles and the wafer will be such that the particles will beattracted and adhere to the wafer surface. This will require that a postCMP clean step remove the adhering particles.

The hydroxylamine or hydroxylamine salts can react with the particlesurface through either a redox reaction or a normal chemical reactionwith the terminal groups on the surface. Since the HDA chemistries canbe chemically "tuned" by adjusting the pH and still be active for metalCMP (see Cu idea above), this will give us a wider process window toaffect the solution slurry Zeta potential. Concentrations for thiseffect should be between 1 to 10 wt % because of HDA's single charge.

Another way to change the Zeta potential is to use surfactants(nonionic, cationic or anion) to reduce the surface charge on the wafer.The hydroxylamine chemistries can be matched with the appropriatesurfactant. Experiments which octylphenol polyethylene (9-10 ethyleneoxide units) at pH 9.5 did reduce the surface tension and also reducedsurface roughness. Anionic surfactants can be used for particles thathave a positive Zeta potentials.

Oxide Polishing: Some of the films currently being planarized includeTEOS, BPSG, PSG and SOG. Though this area of CMP has matured, EKC's HDA(50% hydroxylamine) chemistry with its "buffered" pH of 9.5 to 10.5, andlow mobile ion concentration (Na and K) could be an important newchemistry for the current silicon oxide slurries.

The HDA free base material should be tested at various pH's (7-11) witha silica slurry. The amount of HDA used in the slurry should be ˜2 to10%. SIMS data should show that the mobile ion content remained constantor was decreased.

Though ammonium hydroxide solutions will also polish the siliconsurface, the vapors from the polishing process need to be handled(removed) in an effective manner. The HDA chemistries do not have thesame smell intensity.

Work with ammonium salts added to fumed silica, in the pH range of 6-9for oxide CMP slurry, shows surprising results. Though one expects thehigher pH (˜9) to polish silicon oxide faster (traditional chemicalattack of a base on the Si bond), Hayashi et al. had remarkable successat removing oxide at a pH 6 with a 0.1 molar ammonium salt solution(chloride, sulfate, etc). Even at pH 7 the rate was faster that at pH 9.The results suggest particle agglomeration (change in the electricaldouble layer by modifying the Zeta potential of the fumed silica),forming a "slush" on the particles and the oxide surface. It was alsonoticed that the residual particle count was reduced from 5×10⁵ to 2×10³for a 6" wafer. There is no reason that hydroxylamine salts at this orsmaller concentration ranges should not have a similar effect on thepolishing rate. The pK_(b) 's between the two groups of salts aredifferent which would allow us again to "fine tune" the polishing rates.

One theory is that colloidal silica is very sensitive to pH andundergoes flocculation at pH values near 8, due to the presence ofinsufficient alkali ions.

Ammonium bifluoride is another important ingredient to be evaluated inthe above matrix. Silica dioxide has several solubility regionsdepending on pH. Ammonium bifluoride at low concentrations (>1×10⁻³molar) and low pH (4-6) can be effective for expanding the "window" fordissolving silica structures. This chemistry region might open up anentirely new CMP processing window for ILD. The concentration rangesmust be rather narrow, i.e., 1×10⁻⁵ to 1×10⁻² molar. At higherconcentrations the chemistries start to act as conventional HF etchingmedia (in the pH range 4-7) with very rapid etching.

One important area is the polishing of an oxide/nitride system and beingable to achieve a high oxide to nitride selectivity. Nitride appears toundergoing slow oxidation to a silicon oxide type compound whichundergoes the standard oxide polishing process. This reduces the desiredpolishing selectivity.

Since the HDA free base is a saturated nitrogen solution, and the freebase reacts with oxygen thus creating a solution with very pooroxidizing potentials, it is possible that the nitride structures willnot be readily attacked. Thus the oxide to nitride polishing selectivityshould be enhanced.

Research would also be directed at determining if the HDA solutions arestable under the required CMP conditions and whether there is anenhanced selectivity among various other silicon oxide systems (SOG,TEOS, BPSG, etc.).

Post-CMP Clean: The chemical nature of the wafer surface (hydrophilic orhydrophobic) will effect the method and type of solution necessary toremove particles from the wafer surface after the polishing step. Theparticle charge relative to the wafer surface will determine the type ofchemistry that will effectively remove the particles. Zeta potentials ofthe particles and the effect of the solution pH on this value will needto be understood. Alumina particles can be dislodged under acidconditions but silicon oxide material requires a basic solution.

At the same time it should be advantageous to use solution additives toremove metal contaminates from the wafer surface. Study of residualparticle count and metal contamination levels on wafers from apost-clean procedure allows correlation of this information with the HDAsolution pH and level of additives. These additives will include watersoluble crown ethers and specific metal chelating agents or bufferedcitric acid solutions.

Though HDA and HDA related compounds can effect the particle and wafersurfaces through pH and redox chemistries, these chemical species onlyhave a single ionic charge per molecule (though a reasonable chargedensity for the size of molecule involved). It may be necessary toaugment the electrostatic double layer around the particles or on thewafer by adding "polyelectrolytes" which are highly charged compounds.Normally the polyelectrolytes are used in high enough concentration to"force" particles to clump together. In this invention we only want toadd enough polyelectrolytes encourage the particles to repel each otherand away from the wafer surfaces. This will enhance the post CMPcleaning step. The concentration for this affect could range from 1 partper thousand to 10 wt %.

There are several other types of redox reagents that also can be used inCMP applications which could be used by themselves or in conjunctionwith other chemistries, including hydroxylamine and its salts.

In accordance with another aspect of the invention, ammonium persulfate(ammonium peroxydisulfate) can be used to remove Al, copper or tungstenusing CMP methods. Though ammonium persulfate has been used to stripcopper metal films from electronic component boards, this material hasnot been used to remove Cu in a very controlled manner. We are not awareof this chemistry being used to polish Al metal under CMP processconditions.

The tungsten CMP process appears to operate through the tungstate (WO₄⁼) ion. Though the current CMP processes are based on ferric nitrate orhydrogen peroxide under acid conditions another feasible route to obtainthis species is to oxidize the W metal with an oxidizing agent underbasic conditions. The tungstate should have maximum solubility at pH >6.

Normally ammonium persulfate solutions have a pH in the range of 2 to 3.This invention illustrates that by adjusting the oxidizing solution's pHto higher values, the resulting solution will be a very effective forpolishing W metal films.

EXAMPLES

The following non-limiting examples represent best modes contemplated bythe inventors and describe the invention further. In these examples,solution chemistry was tested as follows:

Example 1

Test: Solutions of ammonium persulfate were prepared and then added to a5% alumina slurry. The pHs were adjusted with NaOH just before use.

The CMP experiments were with 10,000 Å tungsten wafers, at 33 rpm and 2psig. The pad was a Rodell RC 1000 on a Logitech P5M polisher. Base linepolishing experiments with only an alumina slurry have determined thatthere is an 8×to 10× polishing factor between the Logitech and theIPEC/Westech industrial size CMP polisher.

    ______________________________________                                        10% solution  pH 3       removal rate 112 Å                               10% solution  pH 6       removal rate 105 Å                               10% solution  pH 7.7     removal rate 196 Å                               10% solution  pH 7.9     removal rate 198 Å                                5% solution  pH 9       removal rate 176 Å                               ______________________________________                                    

Notice that there appears to be a maximum value at a pH around 7.9.

Example 2

Test: Another composition that was tested was composed of ammoniumpersulfate (APS) with varying concentrations of malonic acid (MA). ThepH was adjusted with sodium hydroxide. Ammonium hydroxide will beoxidized to nitrogen and water.

    ______________________________________                                        APS    MA           pH     Etch Rate (Å/min)                              ______________________________________                                        10%    1%           6      162                                                10%    1%           8.1    460                                                10%    0.4%         8      291                                                 5%    1%           8.8    265                                                10%    0%           8      162                                                ______________________________________                                    

Notice that the best etch rates are seen at pH's above 8 and thatmalonic acid does have a positive effect (10% APS, 0% MA, etch rate 162Å/min), compared to the 5%, 1% MA solution (265 Å/min).

There are other additives that can be added to oxidizers that can alsobe used in the CMP process. These additives can include oxalic acid,lactic acid, gluconic acid, malonamide, and citric acid. These organicacids should have pK_(a) lower than the pH of the planarizationsolution. It is desirable to have these acids in their correspondinganion form, which should be the most effective chelation species.

In addition to malonic acid (HO₂ CCH₂ CO₂ H), APS can be usedeffectively for W CMP when combined with other organic acids: succinicacid (HO₂ CCH₂ CH₂ CO₂ H), tartaric acid (HO₂ CCH(OH)CH(OH)CO₂ H),citric acid (HO₂ CCH₂ C(OH)(CO₂ H)CH₂ CO₂ H), and oxalic acid (HO₂ CCO₂H).

Bases that can be used to adjust the oxidizing solution's pH, includesodium hydroxide, potassium hydroxide, magnesium hydroxide, magnesiumcarbonate and imidazole among others.

There are other potential oxidizer compounds that can be included:

Peroxymonosulfuric acid (Caro's acid) (H₂ SO₅) or its salts are verystrong oxidizing agents, (E^(o) =-1.44V). The acid form has one protonwith a dissociation constant similar to sulfuric acid while the secondproton has a pK_(a) of only 9.4.

Example 3

A commercial product Caroat (potassium peroxomonossulfate compound,including the potassium salt of Caro's acid; empirical formula 2KHSO₆KHSO₄ K₂ SO₄) is a good oxidizer in aqueous system at low pH, butcombined with APS, it shows promising results for W CMP at higher pHvalues. Caroat is a registered product of Degussa Corporation. Thefollowing removal rates are for the Logitech PM5 polisher (33 rpm, 12"IC1000 pad, 2 psig) on 3" wafers (10,000 Åsputtered W), with 5% aluminaslurry (50 parts of 10% alumina +90% water slurry), chemistry additionrate of 100 mL/min, and slurry addition rate of 20 mL/min:

    ______________________________________                                        APS        Caroat              Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                        10         1.0          5.5     90                                            10         1.0          7.5    139                                            10         1.0          8.7    349                                            ______________________________________                                    

Conclusion: Synergism between APS and Caroat enhances W removal rates,with removal rates increasing with increasing pH over the range 5.5 to8.7.

Oxone, peroxymonsulfate has a standard electrode potential similar toperoxymonosulfate, with a wider range of pH stability (between 2-6 andat 12). This material has ˜4.5% "active" oxygen.

Example 4

APS combined with malonamide (H₂ NCOCH₂ CONH₂) shows W removal ratescomparable with those of APS + malonic acid using the Logitech PM5polisher (33 rpm, 12" IC1000 pad, 2 psig) on 3" wafers (10,000 Åsputtered W), with 5% alumina slurry (50 parts of 10% alumina+90% waterslurry), chemistry addition rate of 90 mL/min, and slurry additionalrate of 20 mL/min:

    ______________________________________                                        APS        Malonamide          Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                         5         0            9.0    176                                            10         1.0          9.0    429                                            10         2.5          8.9    385                                            10         2.0          7.9    250                                            10         0            7.9    198                                            ______________________________________                                    

Conclusion: Malonamide enhances the W removal rate when combined withAPS in an aqueous system over the W removal rate of APS alone. Removalrates increase with pH.

Though the use of hydrogen peroxide is well known in the metal CMP fieldit does suffer from poor long term stability when mixed with slurrymixtures. The CMP users have made adjustments to this problem bysegregating the peroxide solution from the slurry until just prior tousage on the polisher. This means that the CMP user must have dualdispensing systems which increases the cost of ownership which directlyaffects the CMP cost per wafer.

In accordance with another aspect of the invention, perborates such assodium perborate tetrahydrate are good compounds which are indirectsources for hydrogen peroxide. The teraborate has a 10.5% active oxygencontent. This compound has a different stability than hydrogen peroxideand therefore could be an important compound for CMP metal etchingapplications. The dry form of the perborate salt is used in manybleaching applications, including detergent formulations, tooth powdersand denture cleaners.

Because of the sodium perborate's low solubility it could also be usedas a slurry or coslurry component. This could be very beneficial to theCMP process since the chemistry is not only acting as an abrasive butalso as an oxidant. Its low solubility but direct contact with themetal/metal oxide could give better etch control.

Other compounds such as sodium carbonate peroxhydrate (2Na₂ CO₃ *3H₂ O₂)contain ˜14 wt % active oxygen. This compound also has a betterstability than hydrogen peroxide and therefore could be an importantmaterial for metal CMP. Test: Experiments with blanket Al metal (5000 Å)wafer showed that a 5 wt % hydroxylamine solution will remove 2 Å/min ofthe metal, but a 5 wt % sodium percarbonate removed 6.4 Å/min. Thepolishing conditions were with a Logitech P5M polisher with a Politexfelt cloth at 33 rpm and 2 psi pressure on the 3" wafer. No slurry wasused during the test.

Example 5

Experiments with blanket W metal (10,000 Å) wafer showed that a 10 wt %hydroxylamine solution will remove 3.3 Å/min of the metal, but a 5 wt %sodium percarbonate removed 168 Å/min. Experiments also showed that a 2wt % ferric nitrate solution will remove only 34 Å/min of metal. Thepolishing conditions were with Logitech P5M polisher with a Politex feltcloth at 33 rpm and 2 psig pressure on the 3" wafer. No slurry was usedduring the test.

In accordance with a further aspect of the invention, another compoundthat will be of interest will be the urea hydrogen peroxide complexwhich will permit a more controlled introduction of the oxidizingchemistry into the slurry system.

Example 6

Experiments with blanket W metal (10,000 Å) wafer showed that a 15 wt %hydrogen perioxide solution with 5% alumina slurry removed 109 Å/min ofthe metal, yet only a 2 wt % urea hydrogen peroxide with only a 2 wt %alumina slurry removed 83 Å/min. It is interesting that a solution seventimes more dilute and less slurry removes almost as much metal as thehydrogen peroxide solution. The polishing conditions were with aLogitech P5M polisher with a Politex felt cloth at 33 rpm and 2 psigpressure on the 3" wafer.

This combination of chemicals will generate environmentally "friendly"waste products (urea and oxygen).

In accordance with still another aspect of the invention, anothercommercially available oxidizing agent that could effective forplanarization tungsten or copper metal is peracetic acid. Thedecomposition products include only oxygen and acetic acid (vinegar).

Test: Experiments with blanket W metal (10,000 Å) wafer showed that a 15wt % hydrogen peroxide solution with a 5% alumina slurry removed 109Å/min of the metal, yet only a 3.5 wt % peracetic acid with only a 2 wt% alumina slurry removed 166 Å/min. It is interesting that a solutionfour time mores dilute and less slurry removes 50% more metal as thehydrogen peroxide solution. The polishing conditions were with aLogitech P5M polisher with a Politex felt cloth at 33 rpm and 2 psigpressure on the 3" wafer.

In accordance with a further aspect of the invention, another uniqueidea is to blend two different chemistries to achieve synergisticinteractions. Two possible chemicals that could be blended are hydrogenperoxide and hydroxylamine.

Example 7

Experiments with blanket W metal (10,000 Å) wafer showed that a 15 wt %hydrogen peroxide solution with a 5% alumina slurry removed 109 Å/min ofthe metal, yet a 10 wt % H₂ O₂ mixed with a 10% hydroxylamine with onlya 5 wt % alumina slurry removed 731 Å/min. The pH was adjusted to 8.7.The polishing conditions were with a Logitech P5M polisher with aPolitex felt cloth at 33 rpm and 2 psig pressure on the 3" wafer.

Example 8

Experiments with blanket W metal (10,000 Å) wafer showed that a 10 wt %hydroxylamine solution will remove 3.3 Å/min of the metal, but a 5 wt %H₂ O₂ and 5 wt % hydroxylamine (pH 7.5) removed 380 Å/min. Experimentsalso showed that a 2 wt % ferric nitrate solution will remove only 34Å/min. of metal. The polishing conditions were with a Logitech P5Mpolisher with a Politex felt cloth at 33 rpm and 2 psig pressure on the3" wafer. No slurry was used during the test.

Another aspect of the invention is to blend two different chemistries toachieve synergistic interactions. Two possible chemical that could beblended are ammonium persulfate and potassium periodate. Potassiumperiodate has a higher oxidation level compared to the potassium iodate.

Example 9

Experiments with blanket W metal (10,000 Å) wafer showed that a 10 wt %ammonium persulfate solution with a 5% alumina slurry removed 162 Å/minof the metal (pH 8), yet a 10 wt % ammonium persulfate mixed with a 2%KIO₄ with only a 5 wt % alumina slurry removed 637 Å/min. The pH wasadjusted to 6.9.

When a 2 wt % potassium iodate (KIO₃) was substituted into the ammoniumpersulfate solution, the polishing rate decreased to 246 Å/min. Thepolishing conditions were with a Logitech P5M polisher with a Politexfelt cloth at 33 rpm and 2 psi pressure on the 3" wafer.

In another aspect of the invention, a similar chemistry to that of theprevious aspect uses a synergism between ammonium persulfate (APS) andperiodic acid (rather than potassium periodate) for polishing tungsten.

Example 10

Removal rates of W generally increase with pH for the periodic acid (H₅IO₆) in water without APS on 3" wafers coated with sputtered W (10,000Å) using 1% or 2.5% alumina (10 or 25 parts of 10% alumina +90% waterslurry), 0-3 parts NH₄ OH to adjust pH, chemistry and slurry combinedtogether at a chemistry/slurry addition rate of 50-100 mL/min, and theLogitech PM5 polisher (33 rpm, 12" IC1000 pad, 2 psig):

    ______________________________________                                        Alumina    Periodic Acid       Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                        1.0        2.0          1.4    130                                            1.0        2.0          1.9    274                                            1.0        2.0          2.1    326                                            2.5        2.0          2.5    252                                            2.5        2.0          6.8    426                                            ______________________________________                                    

Conclusion: tungsten removal rates increase at higher pH values over apH range of 1 to 7 with a constant concentration of periodic acid.

Example 11

Periodic acid in water added to APS increases the removal rate of W overAPS alone at pH 1; increasing the amount of periodic acid used with 10parts APS also increases the W removal rate using the Logitech PM5polisher (33 rpm, 12" IC1000 pad, 2 psig), 3" wafers (10,000 Å sputteredW), 0-3 parts NH₄ OH to adjust pH, 1% alumina (10 parts of 10% alumina+90% water slurry), and chemistry/slurry addition rate of 100 mL/min:

    ______________________________________                                        APS        Periodic Acid       Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                         0         2.0          2.4    130                                            10         2.0          1.1    386                                            10         0.5          3.5    118                                            10         2.0          5.2    388                                            10         0            6      112                                            ______________________________________                                    

Conclusion: There is a synergistic effect that enhances W removal ratewhen APS and periodic acid are used together. Increased removal ratesare observed over a pH range of 1 to 7.

Example 12

Constant removal rates were observed for several days in a periodicacid/NH₄ OH/water system without APS using 0-3 parts NH₄ OH to adjustpH, 2.5% alumina (25 parts of 10% alumina +90% water slurry) added tothe chemistry immediately prior to polishing 3" wafers (10,000 Åsputtered W), chemistry/slurry addition rate of 100 mL/min, and theLogitech PM5 polisher (33 rpm, 12" IC1000 pd, 2 psig).

    ______________________________________                                                     Period Acid                                                                              Removal Rate                                          Time (days)  (parts per 100)                                                                          (Å/min)                                           ______________________________________                                        0            2.0        252                                                   3            2.0        255                                                   ______________________________________                                    

Conclusion: Periodic acid has a very good polishing rate when usedalone, and, unlike hydrogen peroxide, has a good chemical stability overseveral days.

Example 13

A comparison of removal rates for the aqueous periodic acid system isshown below between the Logitech polisher (2 psig) with 3" wafers(10,000 Å sputtered W) and the Strasbaugh 6EC polisher (5-7 psig) with200 mm wafers (10,000 Å sputtered W). Operating conditions were pH 6-7,2.5% alumina (25 parts of 10% alumina +90% water slurry), no APS,chemistry/slurry addition rate of 200 mL/min for the Strasbaugh 6EC(40-50 rpm, 22" perforated IC1000 over SUBA IV pads) and 100 mL/min forthe Logitech PM5 (33 rpm, 12" IC1000 pad). The comparison suggests thatthe removal rates determined used the larger Strasbaugh polisher are 6to 8.6 times larger than those obtained using the smaller Logitechpolisher.

    ______________________________________                                                        Down-   Table          Removal                                Periodic Acid   force   Speed          Rate                                   (parts per 100)                                                                        pH     (psig)  (rpm) Polisher (Å/min)                            ______________________________________                                        2.0      6.8    2       33    Logitech PM5                                                                            426                                   2.0      6      5       40    Strasbaugh                                                                             2535                                                                 6EC                                             2.0      6      5       40    Strasbaugh                                                                             2727                                                                 6EC                                             2.0      6      5       50    Strasbaugh                                                                             3174                                   2.0      6      7       50    Strasbaugh                                                                             3666                                                                 6EC                                             ______________________________________                                    

Conclusion: These results for W polishing show that when comparingremoval rates determined using the Logitech planarizer to largerplanarizers such as the Strasbaugh 6EC, the removal rates must be scaledup by a factor of 6 to 8.6.

Expanding on the last two aspects of the invention, we did a comparisonof polishing rates for the periodate salts potassium periodate (KIO₄)and the lithium periodate (LiH₄ IO₆) as well as potassium iodate (KIO₃)that was used in Wang et al., published PCT Application WO 97 13,889,dated Apr. 17, 1997. The KIO₄ system proved to have higher removal ratesfor W than did the KIO₀₃ system; W removal rates are enhanced whensynergistically combining KIO₄ and APS; and both K and Li periodate maybe used to oxidize W in near-neutral pH regimes, thus getting away fromcorrosion problems associated with very low pH CMP systems. In mixturesof K and Li periodates with APS, systems with higher proportions of Li:Kprovide higher W removal rates.

Example 14

Addition of APS to KIO₃ in water increases the W removal rate, andincreasing amounts of KIO₃ added to APS also increase W removal ratesover a pH range of 5.8 to 7.8 (pH adjusted with 0-3 parts NaOH) usingthe Logitech PM5 polisher (33 rpm, 12" IC1000 pad, 2 psig), 3" wafers(10,000 Å sputtered W), 5% alumina (50 parts of 10% alumina +90% waterslurry), and separate addition of chemistry and slurry with a chemistryaddition rate of 90 mL/min, and slurry addition rate of 20 mL/min:

    ______________________________________                                        APS        KIO.sub.3           Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                         0         2.0          7.0    193                                            10         2.0          7.2    246                                            10         2.0          5.8    208                                            10         5.0          7.2    339                                            10         5.0          7.8    350                                            ______________________________________                                    

Conclusion: Adding APS to KIO₃ increases the W removal rate, increasingpH of the combined APS/KIO₃ /water system increases the W removal rate,and increasing the concentration of KIO₃ in the combined systemincreases the W removal rate.

Example 15

The aqueous potassium periodate (KIO₄) system, with the same polishingparameters as above, also shows a synergistic effect when combined withAPS and shows even a greater removal rate for W than the potassiumiodate system. NaOH (0-3 parts) was used to adjust pH. Operatingconditions included using the Logitech PM5 polisher (33 rpm, 12" IC1000pad, 2 psig), 3" wafers (10,000 Å sputtered W), 5% alumina (50 parts of10% alumina +90% water slurry), chemistry addition rate of 90 mL/min,and slurry addition rate of 20 mL/min:

    ______________________________________                                        APS        KIO.sub.4           Removal Rate                                   (parts per 100)                                                                          (parts per 100)                                                                            pH     (Å/min)                                    ______________________________________                                         0         0.2          7.9    142                                            10         0.2          7.7    405                                            10         2.0          6.9    637                                                       (supersaturated                                                               solution)                                                          ______________________________________                                    

Conclusion: Synergism between APS and KIO₄ enhances W removal rates atnear-neutral pH.

Example 16

Mixtures of Li and K periodate show improved removal rates for higherproportions of Li:K. There is also an effect of pH noted in the tablebelow: increased removal rate with increasing pH. Polishing parametersare for the Logitech PM5 polisher (33 rpm, 12" IC1000 pad, 2 psig), 3"wafers (10,000 Å sputtered W), 1% alumina (10 parts of 10% alumina +90%water slurry), and chemistry/slurry addition rate of 100 mL/min:

    ______________________________________                                        APS      LiH.sub.4 IO.sub.6                                                                       KIO.sub.4       Removal Rate                              (parts per 100)                                                                        (parts per 100)                                                                          (parts per 100)                                                                          pH   (Å/min)                               ______________________________________                                        10       0.4        0.0        7.2  382                                       10       0.3        0.1        7.2  215                                       10       0.2        0.2        6.5  175                                       10       0.1        0.3        6.1  170                                       ______________________________________                                    

Conclusion: Addition of Li and/or K periodate to an aqueous APS systemenhances W removal at near-neutral pH. In mixed Li/K periodate+APSsystems, higher proportions of Li:K provide higher W removal rates atnear-neutral pH.

Example 17

Tungsten removal rates using the 10 parts APS +0.4 parts Li periodateare stable for a period of several days when combined with aluminaslurry. The pH was not adjusted, but stayed near-neutral, between pH 6.4and 7.6, during the course of the test. Polishing was done used theLogitech PM5 polisher (33 rpm, 12" IC1000 pad, 2 psig), 3" wafers(10,000 Å sputtered W), 5% alumina (50 parts of 10% alumina +90% waterslurry), and chemistry/slurry addition rate of 100 mL/min:

    ______________________________________                                        Time (days)  Removal Rate (Å/min)                                         ______________________________________                                        1            208                                                              7            244                                                              15           218                                                              ______________________________________                                    

Conclusion: Even when combined with alumina slurry, the APS/LiH₄ IO₆/water system has high and stable removal rates for more than 2 weeks,providing a better shelf life than the acidic ferric nitrate/wateralumina system which must be combined at point-of-use.

Example 18

A quantity of 500 ml of two comparative chemical solutions was eachplaced in a 600 ml beaker equipped with a magnetic stirring rod. Thefirst ammonium persulfate solution consisted of 114 parts of ammoniumpersulfate in deionized water to give total of 1000 parts of solutionhaving a pH of 3.1. The second ferric (III) nitrate solution consistedof 40 parts of ferric (III) nitrate nanohydrate dissolved in deionizedwater to give a total of 1000 parts of solution having a pH of 1.5.These solutions were tested with silicon wafers at room temperature asfollows:

Three inch wafers with a 300 Angstroms (Å) Ti metal adhesion layer and3000 Å0 sputtered Cu were used. At selected time intervals, the wafersample was removed, rinsed with DI water and then dried with nitrogengas. A conventional four point probe was used to determine the metalfilm thickness. The etch rates were:

Ammonium persulfate 3000 Å/min

Ferric (III) nitrate 1287 Å/min

One would have expected the chemistry with the lowest pH (more acidic),i.e., the ferric (III) nitrate solution, to etch the Cu the fastest.

Example 19

In this series of tests, the effectiveness of hydroxylamine nitrate atvarious pH levels was tested for etch wafers with 3000 Å sputtered Cuand a 300 Å Ti adhesion layer. The apparatus was as used in Example 1.The solution was composed of 24 parts by weight of 82 weight percenthydroxylamine nitrate in 176 parts by weight of DI water. The pH wasadjusted with small quantities of hydroxylamine, as the free base. Thehydroxylamine free base was composed of 20 parts by weight of itscommercially available approximately 50 percent by weight aqueoussolution and 80 parts by weight deionized water. Also used was anammonium hydroxide solution composed of 80 parts by weight of a 25percent by weight aqueous ammonium hydroxide solution and 120 parts byweight of deionized water.

After a certain interval, the wafer was rinsed with deionized water anddried with nitrogen. The wafer was then weighed. A separate blank Tiwafer was etched in a 10 percent by weight H₂ O₂ solution to determinethe amount of Cu on each 3 inch wafer. The results obtained are shown inthe table below.

    ______________________________________                                        Chemistry         pH     Etch rate (Å/min)                                ______________________________________                                        Hydroxylamine nitrate                                                                           3      120                                                  Hydroxylamine nitrate                                                                           4      150                                                  Hydroxylamine nitrate                                                                           5      600                                                  Hydroxylamine (free base)                                                                       11.7    75                                                  Ammonium hydroxide                                                                              12.7   100                                                  ______________________________________                                    

It is well known that Cu metal will be etched with inorganic and organicamines at pHs above 9. It is also known that Cu metal will be etched atvery low pHs (below 3). The above results are quite surprising, since asignificant etch rate was seen at ph 5.

In a further aspect of the invention, other chemistries that have givengood CMP process results are based on hydroxylamine nitrate (HAN) andother hydroxylamine salts. Besides several examples with HAN, oneexample examines the use of citric acid in combination with HAN. Othercombinations could include mono-, di- and tri- organic acids. Examplesof such acids include, but are not limited to acetic acid, malonic acidand citric acid, respectively.

Example 20

Amines (and ammonia compounds) are more effective in neutral or basicsolutions for polishing (etching) copper. Some ammonium compounds haveonly moderate success at polishing copper at low pHs. Hydrogen peroxidechemistries are usually used at low pHs. The following example showsthat hydroxylamine nitrate (HAN, a mild oxidizing agent) willeffectively polish copper. Hydroxylamine and its salts are not aminesbut do contain the NH2-group found in inorganic and organic amines.Hydroxylamine's NH2-group is attached to a hydroxyl (HO-group) which isnot found in "amines" and does influence its oxidation-reductionpotential.

These results were obtained by immersing a copper wafer (10,000 Å) instirred 10% hydroxylamine nitrate solutions (12.2 parts of 82% HAN in87.8 parts water) for various time periods. At certain time periods thewafers were removed, rinsed with DI water, dried with nitrogen and thenweighed to the nearest 0.1 mg. Another wafer from the same group wasetched with an ammonium peroxydisulfate solution (10 partsperoxydisulfate and 90 parts water) until there was no further weightloss. It was possible to use weight ratios to determine the metal lossin Å/min. The hydroxylamine nitrate results were compared to a 10%ammonium hydroxide solution (10 parts 27% ammonium hydroxide in 90 partswater) under similar conditions.

    ______________________________________                                        Etchant         pH     Removal Rate (Δ/min)                             ______________________________________                                        Hydroxylamine Nitrate                                                                         3.1    120                                                    Hydroxylamine Nitrate                                                                         4.0    150                                                    Hydroxylamine Nitrate                                                                         5.0    600                                                    NH.sub.4 OH     12.7   100                                                    ______________________________________                                    

This example shows that hydroxylamine compound will remove copper metaland that there is a definite optimum pH. The ammonium hydroxide had thepoorest etch rate even though this is an optimum pH region for etchingcopper with amines.

Example 21

In this example the hydroxylamine nitrate chemistry is used in aslurriless polishing system. A Logitech PM5 polishing system (used forCMP modeling experiments) was used with a Politex felt pad at 33 rpmwith 2 psig pressure on the 3" copper wafer. The 5% chemistry (6.1 partsHAN with 95.9 parts water) was added to the polishing table at 50mL/min. The removal rate was determined by a Four Dimensions four pointprobe used for determining metal film thickness on wafers.

    ______________________________________                                        pH         Removal Rate (Å/min)                                           ______________________________________                                        4.2         18                                                                6.0        218                                                                ______________________________________                                    

This example shows that there is a pH effect with the HAN solutions. Themetal film had a very bright finish.

Example 22

In this example a 10% hydroxylamine nitrate solution (12.2 parts of HANin 87.8 parts water) mixed with a 2.5% silicon oxide slurry was usedwith a Politex pad on the Logitech PM5 polisher was 33 rpm with 2 psigpressure on the 3" copper wafer. The chemistry was added to thepolishing pad at 90 mL/min. The removal rate was determined by a FourDimensions four point probe for determining metal film thickness onwafers.

    ______________________________________                                        pH         Removal Rate (Å/min)                                           ______________________________________                                        2.6        1270                                                               4.0        1014                                                               ______________________________________                                    

This example shows that the use of a silicon oxide slurry will shift theeffective polishing rate to very low pHs with very good copper removalrates. This example also shows that the HAN chemistry works well withslurries with the Logitech modeling equipment. The metal film had a verybright finish.

Example 23

In this example a commercial alumina slurry is used with variouschemistries. The slurry concentration was 2.5% used with a Politex padon the Logitech PM5 polisher at 33 rpm with 2 psig pressure on the 3"copper water. The hydrogen peroxide solution was composed of 15 parts ofa 30% H₂ O₂ solution mixed with 85 parts of water.

    ______________________________________                                                    pH   Removal Rate (Å/min)                                     ______________________________________                                        5% HAN        5      950+                                                     5% HAN        5      950+                                                     5% HAN        6      575+                                                     15% H2O2      4      65                                                       H2O           4.8    44                                                       ______________________________________                                    

This example shows that the polishing rate for HAN is reproducible andis polishing better than the traditional hydrogen peroxide chemistry forcopper CMP. The water experiment shows that the copper polishing rate isnot solely a pH effect. The metal films polished with HAN had verybright finishes, but the hydrogen peroxide polished wafer was "cloudy"and the water polished wafer was dull.

Example 24

Another important feature is a good shelf life after the slurry andchemistry are mixed together. Currently the hydrogen peroxide/slurrysystems are so unstable that the industry currently mixes the slurry andthe chemistry only at the point of use. Premixed hydrogenperoxide/slurry solutions only have several hours of useful life.

In this example a 0.5 wt % hydroxylamine nitrate solution (0.6 parts ofHAN in 99.4 parts water) mixed with a 2.5% alumina slurry. A masterbatch was made and stored in a plastic container. Samples of thechemistry/slurry were then removed after certain number of days and usedin the polishing experiment. The pH of the slurry varied only between 4and 4.1 during the 22 day trial. The slurry mixture was used with aPolitex pad on the Logitech PM5 polisher at 33 rpm with 2 psig pressureon the 3" copper wafer. The chemistry was added to the polishing pad at50 mL/min. The removal rate was determined by a Four Dimensions fourpoint probe for determining metal film thickness on wafers.

    ______________________________________                                        Day        Removal Rate (Å/min)                                           ______________________________________                                        0          637                                                                4          1064                                                               22         558                                                                ______________________________________                                    

Except for the fourth day result which increased by ˜40%, the 22nd dayresult clearly shows that the chemistry is still giving good polishingrates. The metal films had very bright finishes.

Example 25

Another feature is the selectivity of the polishing rate betweendifferent materials on the wafer. It is important that all materials(metals and the surrounding IDL layers) are not polished at the samerate, otherwise it would be difficult to stop at a specific layer.

The following example shows the selectivity between the copper metal anda BPSG film. In this example a 0.5 wt % hydroxylamine nitrate solution(0.6 parts of HAN in 99.4 parts water) mixed with a 2.5% alumina slurry.The pH of the slurry varied between 4 and 4.4. The slurry mixture wasused with a Politex pad on the Logitech PM5 polisher at 33 rpm 2ith 2psig pressure on the 3" copper wafer. The chemistry was added to thefour point probe for determining metal film thickness on wafers, and theBPSG film thickness was determined by ellipsometer.

The copper film removal rate was 637 Å/min while the BPSG film was onlypolished at a 37 Å/min rate. The selectivity of Cu to BPSG was 17.2.This means that the polishing process will "stop" when the BPSG layer isreached, since it has a much slower polishing rate.

In a further aspect of the invention, another way to polish copper is touse a combination of chelating agents (polyfunctional organic acids)with the conjugate hydroxylamine salts.

Example 26

In this example a solution of citric acid (8.8 parts citric acidadjusted with hydroxylamine to a pH 4.2 to 4.4, the remainder is water)is mixed with various concentration of hydroxylamine (HDA) to obtainsolutions with pHs close to neutral. These chemistries were used in aslurry polishing system. A Logitech PM 5 polishing system was used witha Politex felt pad at 33 rpm with 2 psig pressure on the 3" copperwafer. The chemistries were added to the polishing table between 20 to90 mL/min. The removal rate was determined by a Four Dimensions fourpoint probe for determining metal film thickness on wafers.

    ______________________________________                                        Parts Citric Acid Sol.                                                                     Parts HDA pH      Removal Rate (Å/min)                       ______________________________________                                        100           0        4.2      58                                            95            5        6.6      64                                            90           10        6.8     954                                            80           20        7.0     1100                                           ______________________________________                                    

This example shows that even though the pH is only varied over a 0.4 pHrange (for the HDA salt solutions) there was a significant increase inthe copper etch rate, related to the to increase in the hydroxylaminesalt of the citric acid.

The invention now being fully described, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of theinvention.

We claim:
 1. In a composition for chemical mechanical polishing, theimprovement wherein said composition comprises an effective amount forchemical mechanical polishing of a hydroxylamine compound and furthercomprises octylphenyl polyethylene.
 2. In a composition for chemicalmechanical polishing, the improvement wherein said composition comprisesan effective amount for chemical mechanical polishing of a hydroxylaminecompound and said composition further comprises ammonium bifluoride. 3.In a composition for chemical mechanical polishing, the improvementwherein said composition comprises an effective amount for chemicalmechanical polishing of a hydroxylamine compound and said compositionfurther comprises a polyelectrolyte.
 4. In a composition for chemicalmechanical polishing, the improvement wherein said composition comprisesan effective amount for chemical mechanical polishing of a hydroxylaminecompound and said composition further comprises hydrogen peroxide.
 5. Acomposition for chemical mechanical polishing, which comprises a slurry,a sufficient amount of hydroxylamine or a hydroxylamine salt to producea differential removal of a metal and a dielectric material, a pHadjusting compound to adjust the pH of the composition to provide a pHthat makes the selectively oxidizing and reducing compound provide thedifferential removal of the metal and the dielectric material, andammonium peroxydisulfate.
 6. A composition for chemical mechanicalpolishing, which comprises a slurry, a sufficient amount ofhydroxylamine or a hydroxylamine salt to produce a differential removalof a metal and a dielectric material, a pH adjusting compound to adjustthe pH of the composition to provide a pH that makes the selectivelyoxidizing and reducing compound provide the differential removal of themetal and the dielectric material, and an oxidant chosen from the groupconsisting of potassium periodate, lithium periodate, potassium iodateor periodic acid.